When the one doc showed the commercial bee farmer delivering his hundreds of bee hives to an almond orchard/farm (IIRC) and then the almond farmer sprayed the trees with all sorts of chemicals while the bees were on the trees it didn't take much more to figure out why his bees died in droves.
Seriously, how was this ever considered a mystery?
Is any of this killing the Africanized killer bees?
Are hostile invasive species responsible for some of the lowering numbers?